Optical Transport Network (OTN) is a set of standards defined by the ITU-T for transmission of optical signals over fiber using wavelength division multiplexing (WDM). These standards cover the physical layer, signal rate, format specification, equipment functional requirements and OAM&P (Operation, Administration, Maintenance and Provisioning) for the network.
OTN provides a method to transport a client signal such as an STM-16/OC-48 or a 10G Ethernet or a storage area network (SAN) signal efficiently over fiber. Multiple client signals can be carried in the same fiber using time-division multiplexing (TDM) and WDM. OTN is significantly less complex compared to SONET/SDH and this reduced complexity offers optimized overhead and substantial reduction in costs. Further, the multiplexing bandwidth granularity is one or two orders of magnitude greater than SONET/SDH making OTN much more scalable to higher rates. It also provides an integrated forward error correction (FEC) mechanism that allows for greater reach between subsequent nodes and higher bit rates on the same fiber. This, too, adds to reduction in costs.
The Optical Channel Payload Unit (OPU) is the basic container that carries the client signal. The client signal is mapped into the payload area of the OPU using either the Asynchronous Mapping Procedure (AMP) or Generic Mapping Procedure (GMP). It is a set of overhead bytes containing information to support adaptation of the client signal. This overhead is terminated wherever the client signal is assembled or disassembled. The payload area of the OPU consists of 4 rows×3808 columns (columns 17-3824). The overhead area consists of 4 rows×2 columns (columns 15-16).
The Optical Data Channel Unit (ODU) consists of the OPU and an ODU overhead area. The ODU overhead area is 4 rows×14 columns (columns 1-14). Note that row 1 of this overhead area is reserved for the frame alignment bytes and the Optical Channel Transport Unit (OTU). The ODU overhead contains information for the maintenance and operational functionality to support optical channels. It also has provision for end-to-end path monitoring of the ODU path and up to six level of tandem connection monitoring. The overhead is terminated wherever the ODU is assembled or disassembled.
The Optical Channel Transport Unit (OTU) conditions the ODU for transport over an optical channel network connection. The OTU consists of a Framing Alignment (FA) overhead, OTU overhead and a FEC. The FEC is 4 rows×256 columns (columns 3825-4080). The FA overhead is a fixed standardized pattern that is used to detect the start of the OTU frame and a one byte multi-frame alignment signal to indicate the multi-frame number of the current OTU frame. The OTU overhead contains information for operational functional to support the transport via one or more optical channel connections. This overhead is terminated wherever the OTU is assembled or disassembled.
The following explains OTN Multiplexing Structure. At the ODU level, OTN supports a single level of multiplexing. The higher order ODU is designated as ODUk and the lower order ODUs contained within the ODUk as ODUj. The ODUk is divided into timeslots of 1.25 G (ODU0 capacity) in case of equipment adhering to the newer standard, while older equipment uses 2.5 G (ODU1 capacity) timeslots. The Table-I below illustrates the number of timeslots each ODU has.
TABLE IODUk type2.5G timeslots1.25G timeslotsODU112ODU248ODU31632ODU4—80
Thus, an ODU2 may contain a maximum of 4 ODU1s or 8 ODU0. An ODU3 may contain a maximum of 16 ODU1, 4 ODU2 or 32 ODU0 and so on. The ODUk may contain a mix of ODUjs within the limit of the capacity of the ODUk.
The multiplexing of the ODUjs within the ODUk is done based on interleaving timeslots. FIG. 1 illustrates multiplexing of the ODUjs within the ODUk for k=2 and ODUj (j=1). The ODU1 overhead for each of the ODU1s repeats every four ODU2 frames. This is locked to bits 7 and 8 of the MFAS byte. Hence, for each ODU1 that is multiplexed as part of the ODU2 may have a justification opportunity every four ODU2 frames.
For inter-operating with older equipment that uses the 2.5 G timeslot scheme with newer equipment that use the 1.25 G timeslots, standards define that an ODU1 that is multiplexed into ODU2 uses the ‘i’ (i=1, 2, 3, 4) and the ‘i+n’ (n=4) timeslots. This ensures that the receiver would see the ‘i’th ODU1 in every fourth column of the ODU2 payload area. Similarly for ODU3 i=1, 2 . . . 16 and n=16. This takes care of the overhead repeating exactly once every four frames.
The complete multiplexing hierarchy for ODUk through ODUj is shown in the FIG. 2.
In OTN based equipments it's a common requirement to map/de-map lower order ODUjs inside higher order ODUk's using an ODU connection function to route, groom and multiplex the ODUjs clients in an ODUk server.
In some of the HO ODUk's a mix of different LO ODUjs are allowed. If a restriction is imposed on the timeslot usage for a particular LO ODUj, it can lead to possible fragmentations at time slot level depending on the order of connections created. To avoid such fragmentations and ensure usage of complete bandwidth available in ODUk, the ODUjs are allowed to be flexibly mapped into any of the timeslots available.
The SDH systems have a connection model centered on connections identified by STM port number and timeslots S, K, L and M. Similarly, in the current state of the art, for OTN networks too the connection model is centered on time-slots occupied by the LO ODUj inside the HO ODUk. As illustrated in FIG. 3, the ‘A’ and ‘Z’ ends for an ODUj trail agree upon the set of time-slots which the ODUj will occupy inside the ODUk. The time-slots used at the originating end for the ODUj (‘A’) and those used at the receive end (‘Z’) hence must be the same. This agreement henceforth will be referred to as “policy” for a connection. Similarly, the “identity” of the ODUj in the network is based on the set of time-slots with which it's associated.
At either of the end-points of the network connection, the child ODUj (as a client) inside parent ODUk (as a server) is identified as follows:                ODUj identifier=[ODUk identifier],                    [Set of timeslots in HO ODUk]                        
Since, the identity for the ODUj involves a set of time-slots, the ‘A’ and ‘Z’ ends have to agree upon their respective transmit (at ‘A’ end) and receive time-slots (at ‘Z’ end) if the intended ODUj trail is the transport entity being connected end-to-end.
As an example ODU3 is taken as an example of HO ODUk and ODU2 as a LO ODUj. An OPU3 (one to one mapped with the ODU3) when partitioned into 2.5 G timeslots looks as shown in FIG. 4 (taken from ITU-T G.709/Y.1331).
As shown in the FIG. 4 there are 16 tributary time-slots in each OPU3 frame. If one has to multiplex 4 ODU2s inside this OPU3 there is a flexibility provided in terms of each ODU2s taking any set of 4 timeslots out of these 16 timeslots. FIG. 5 shows the multiplex structure being carried in the PSI bytes over an ODU3 multi-frame.
The tributary port numbers (TPNs) are a way to group the time-slots and convey which single ODUj they conform to. The TPN to TS mapping is carried in the multiplex structure identifier (MSI). The ODTU type tells if the time-slot is carrying an ODTU13 or ODTU23.
The ODTU type field takes different meanings based on the OPUk type; here it's shown for an OPU3. For example for OPU4, the ODTU type field is absent and instead there's a 1 bit field which just says “allocated” or “unallocated”.
As an example, when multiplexing 4 ODU2s in an OPU3 (mapped into ODU3 finally), the TPN values range from 1 to 4 with each value representing a single ODUj.                [TPN=1] represents an ODU2 in [TS=1, 10, 11, 16]        [TPN=2] represents an ODU2 in [TS=2, 4, 6, 8]        [TPN=3] represents an ODU2 in [TS=3, 13, 14, 15]        [TPN=4] represents an ODU2 in [TS=5, 7, 9, 12]        
So, as an entity identification of an ODU2 on end-to-end basis one can set a policy between A and Z ends as shown in FIG. 3 that states as follows:
‘A’ end will transmit an ODU2 in question on [TS=1, 10, 11, 16]
‘B’ end must receive the same ODU2 in question on [TS=1, 10, 11, 16]
In this case, if one end has to setup a connection in the network, that end is expected to choose these 4 ODU2 time-slots inside the ODU3. This is a traditional method by specifying the time-slots.